1. Technical Field
The present invention relates to a method for polishing an object such as a semiconductor wafer using an abrading plate to obtain a flat and mirror finish.
2. Background Art
Recently, as the density of circuit integration in semiconductor devices becomes ever higher, circuit patterns are becoming finer and interline spacing narrower. Especially, since line width becomes finer, the depth of focus of a stepper becomes very shallow in photolithographic reproduction of circuit patterns, and the surface of the wafer placed at the focal plane of the stepper must be flat to produce the required degree of image sharpness. A method of obtaining a flat surface is to polish the object in a polishing apparatus.
A type of conventional polishing apparatus comprises a turntable having a polishing cloth and a top ring which is pressed against the turntable with a given pressure while the polishing object is held therebetween, and supplying a polishing solution to the interface to produce a flat and mirror polished surface. This approach is called chemical mechanical polishing (CMP).
FIG. 1 shows essential parts of an example of a conventional CMP apparatus. The apparatus is provided with a rotating turntable 5 having a polishing cloth 6 such as a urethane cloth; a top ring 1 holding an object such as a semiconductor wafer 4 against the cloth 6; and a spray nozzle 9 for supplying a polishing solution Q to the polishing cloth 6. The top ring 1 is connected to a top ring shaft 8, and the top ring 1 holds the semiconductor wafer 4 in contact with an elastic mat 2 such as polyurethane. The top ring 1 has a cylindrical guide ring 3 on its outer periphery so that the wafer 4 will not disengage from the bottom of the top ring 1. The guide ring 3 is fixed to the top ring 1, and the wafer 4 is held within the holding surface so that the wafer 4 will not jump outside of the top ring due to frictional forces with the cloth 6. The top ring 1 is supported on a spherical bearing 7 so that it can be tilted with respect to the shaft 8.
While holding the wafer 4 in the bottom surface of the elastic mat 2 of the top ring 1, the wafer 4 is pressed against the cloth 6 on the turntable 5, and the turntable 5 and the top ring 1 are independently rotated so as to slide the surface of the wafer 4 relative to the cloth 6 to polish the wafer 4. In this case, a polishing solution Q is supplied from the nozzle 9 to the top surface of the cloth 6. The polishing solution comprises abrasive particles, for example, such as silica particles, suspended in an alkaline solution, which have two effects of CMP for semiconductor wafer, chemical polishing using an alkaline solution, for example, and mechanical polishing using abrasive particles.
However, the conventional polishing methods of CMP using such a slurry solution containing numerous abrasive particles present two operational problems.
The first problem is that, during the initial stage of polishing, raised regions of the surface structure are preferentially removed, but gradually, depressed regions are also removed. Therefore, irregularities of the surface are difficult to decrease. It is considered that this phenomenon in CMP is created because a relatively soft cloth is used along with freed abrasive particles contained in the polishing solution, so that not only the raised portions but also the depressed portion of the surface structure are also removed by such abrasive particles. FIG. 2 illustrates such problems of the conventional CMP, which shows irregularities caused by a raised portion and depressed portion of surface film thickness structure on the vertical axis and relative time on the horizontal axis. As indicated in this graph, at a relative time of 1 to reach a stage of surface removal, the raised regions are polished from a height of about 27,000 angstroms to a height of 16,000 angstroms, but the depressed regions are also polished from 20,000 angstroms to 16,000 angstroms, at which point, the irregularities are eliminated. FIG. 3A shows surface profiles of a fine surface structure in an initial stage, FIG. 3B in a middle stage, and FIG. 3C in a final stage of polishing. As illustrated in these drawings, the irregularities are very difficult to remove, and consequently, it is a time-consuming operation.
The second problem relates to cost and environmental considerations. The polishing solution is usually a slurry containing a fine silica powder in suspension, but to obtain a uniformly flat surface of high quality, it is necessary to supply the solution in a liberal quantity to the polishing surface. However, most of the solution supplied is actually discharged as a waste solution without contributing much to the polishing process. Polishing solutions used for high precision polishing of semiconductor devices are expensive, which is a factor leading to a problem of high polishing cost. Also, because such polishing solution in a slurry condition contains a large quantity of abrasive particles such as silica particles, the maintenance of working environment is apt to be difficult. That is, contamination of a solution supply system and a drainage system by the slurry is serious, and the waste solution must also be treated extensively before discarding. Also, after a CMP process, the wafers are washed to remove the polishing solution, but the waste water from this operation also must be treated in a similar manner and poses an environmental problem.